Posts tagged 3D scanning

Revision of scan results

As promised, we would give the summarized results for the two scanning sessions that we had: with CT scanners and with Artec Spider scanner.

Part of the results of the latter unexpectedly disappeared during the post-processing. The scan which promised the most for us, Hermione handle detail, was among the missing files. We hoped to get better results of the floral ornament and combine it with the CT-scan body, only the chosen comparison model and Harry survived.

Kam artec spider

As seen, for the comparison we chose the finest model we had. The lice-comb teeth were approximately 0.5 mm diameter with even smaller gaps between them. Due to this, the scanning technique used by Artec Spider could never achieve a proper result: too much was not visible, even with the precision of 0,05 mm. In other words, the grid which the range finding device projected could not be interpreted in the gaps and the result was a block with a texture instead of a comb. Moreover, looking at he scan of Harry we can see a big inconvenience for us: only the outer surface and the sections at the breaks were captured. Moreover, the cracked surface texture was not captured, because we got the file only in a mesh file.

artec scanner harry

Therefore, it would be expected that this problem would not be so apparent in CT-scans. This technique captures the sections of the object, instead of making an interpretation of surface. Just then these sections are interpreted into 3D files. However, the precision of 0,3 mm proved to be insufficient for the artifact we chose:

macro CT kam

As seen, the result was a more consistent file, which could actually be printed. Nevertheless, it was far from what we would call sufficient. Expecting this, we also made micro-CT scans of the object. The sneak-peaks of the object in the lab itself looked very promising. Yet our and publicly available computers could not handle the size of the data set (over 2000 sections!) and could only give results in the lowest resolution, leaving us with the following model:

micro kam

As seen in the picture, the separate teeth are clearly visible, although the main body is missing. This can be easily solved if the used computer has 16GB RAM, since we could get a proper model in Avizo a few moments before it crashed due to memory insufficiency.

To conclude, only the micro-CT scanner offered the sufficient results for the compared artifact. The Artec Spider is very interesting if surface detailing in necessary or if textures/colors have to be captured. However, if not enough scans are made and combined, you will get an object lacking details, thus resulting in incredible amount of work hours in post-processing. Another solution for this would be to make CT-scans and combine them with the Artec Spider scans only for the details.

CT-scanning and priorities

As the project slowly went into motion we had the first digitizing session in the laboratory of Geosciences&Engineering. Our group was provided with the luxury to first hand observe both micro- and macro-CT scanners in working. Both with their advantages and limitations, they gave us a new perspective of how to order and process given archaeological objects.

When Maaike came in with boxes full of ceramics from the Archaeological archive of Amsterdam, we understood that it was neither efficient, nor possible to scan them all. At this point selection was crucial. At first sight we had three main groups of objects: lice combs (highest level of detail), broken colored ceramics bound with metal strings (necessity to make more detailed scans to understand the technique) and sets of white ceramic tableware lacking multiple shards.

DSC_0381DSC_0307Detail hermione

The latter seemed to be the closest to the issues visible in the goal of the project. Yet the other two gave us interesting side paths which would improve overall understanding of the methods and possibilities of 3D scanning. Based on this, we made a queue sorted by importance, which would lead to at least one object of a group scanned.

DSC_0292

After the first inspection of the digitized forms we were rather amazed that the precision of 0,3mm was not sufficient for some of the fine-detailed specimens. E.g. the combs lost their teeth, metal bindings were muffled, crack lines barely visible. Consequently we were offered to work with much finer machinery (micro-CT scanner) mostly used for small scale material research. Yet the time and money needed for this method led to only two specimens scanned: the finest ivory comb and a detail of a metal connection. In total we got 13 scans, excluding identical scans in higher precision. The notes and conclusions after this are as follow:

1. There are 2 CT-scanners in the Geoscience&Engineering laboratory:

  • Macro-scanner can be used to scan rather big objects, but the fine details are almost completely neglected; object is stationary, thus there is a small chance of damage. Precision 0,3mm.
  • Micro-scanner is very slow (1h per object) and has very limited object size: till 100-120mm in diameter; object is rotating, thus it needs to either be glued or fixed, which requires extra attention not to damage the object. Precision 0,03mm.
  • Both scan only the form and not color; they can detect cavities, but not slight changes in the material density

2. The digitized forms are saved as 2D images of section cuts in .dcm or .ima file format, which need multiple steps to be converted into editable 3D objects. Even though we were informed that it is a very quick procedure, to gain fine details it is necessary to have a powerful computer(16GB RAM) and correct software (which is usually paid).

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